1. Field of the Invention
The present invention relates to a sulfonated polyphosphazene derivative, which is a proton conductor capable of being adopted, for example, as a solid polymer electrolyte membrane or a solid polymer ion exchange membrane for a fuel cell, and a method for producing the same.
2. Description of the Related Art
A proton conductor such as a perfluorosulfonic acid polymer membrane is used, for example, as a material for an electrolyte to be interposed between an anode electrode and a cathode electrode of a fuel cell. The hydrogen ion (proton), which is produced by the ionization of hydrogen contained in a fuel gas supplied to the anode electrode, is moved by the electrolyte to the cathode electrode. Such movement occurs even at a relatively low temperature.
The proton conductivity of the perfluorosulfonic acid polymer membrane is lowered, as the membrane becomes drier. Therefore, when the fuel cell is constructed by using an electrolyte composed of a perfluorosulfonic acid polymer membrane, for example, steam is contained in the fuel gas to be supplied to the anode electrode and/or the oxygen-containing gas to be supplied to the cathode electrode, so that the membrane is continuously replenished with water. However, in this case, the system is large-scaled, because the fuel cell is additionally equipped with a humidifier for producing steam.
Sulfonated polyphosphazene is exemplified as another proton conductor by Mark V. Fedkin et al. in “Evaluation of methanol crossover in proton-conducting polyphosphazene membranes,” Material Letters, January 2002, Vol. 52, pp. 192-196. Sulfonated polyphosphazene is known as a cation exchange resin. The larger the ion exchange capacity is, the larger the moisture-absorbing ability becomes, in the same manner as general cation exchange resins. In other words, in this case, the ability to absorb water in atmospheric air, and water produced by operation of the fuel cell, are enhanced. Therefore, it is possible to ensure proton conductivity without providing any additional humidifier. However, the ion exchange capacity of sulfonated polyphosphazene is about 1.1 to 1.6 mmol/g, which is somewhat low, and its moisture-absorbing ability is poor.
On the other hand, a production method is disclosed in “Sulfonated polyphosphazene ion-exchange membranes,” Journal of Membrane Science, 1996, Vol. 119, pp. 155-160, written by Ryszard Wycisk et al. In this method, anhydrous sulfuric acid is added within a range in which SO3 of anhydrous sulfuric acid is 0.6 to 1.9 in molar ratio per repeating unit of a polyphosphazene derivative, followed by performing a reaction at room temperature for 3 hours to obtain a sulfonated polyphosphazene derivative. In this document, it is reported that the sulfonated polyphosphazene derivative, which has a maximum ion exchange capacity of 3.0 mmol/g, is obtained.
However, in the case of the method described in this document, not only the phenyl group as a side chain of the polyphosphazene derivative, but also the N═P bond for constructing the principal chain is also sulfonated during the process to effect the sulfonation. For this reason, the principal chain is dissociated, and the number average molecular weight of the polyphosphazene derivative is decreased. In a solid polymer type fuel cell, gas pressure is allowed to differ between both electrodes in order to improve power generation efficiency in some cases. In such a situation, durability is deteriorated with the membrane being composed of a polymer having a small number average molecular weight.
The maximum theoretical value of ion exchange capacity is 4.9 mmol/g when the two phenyl groups are sulfonated and N of the principal chain is not sulfonated. However, as of yet, no sulfonated polyphosphazene derivative has been known having an ion exchange capacity exceeding 3 mmol/g.
That is, it is difficult to increase both ion exchange capacity and the number average molecular weight of a sulfonated polyphosphazene derivative. For this reason, no sulfonated polyphosphazene derivative has yet been obtained, which is excellent in moisture-absorbing ability together with exhibiting high proton conductivity.